Capital Bikeshare has, by most any reasonable measure, been a rousing success. And there’s a ton of great anecdotes out there as to how it is bringing about a sea-change in bicycling (my favorite bike convert story leads this post). But with slightly less than two years in service, there’s not a whole lot of evidence out there on the impact CaBi is having on bicycling in general in the DC area. The most basic question is, “how much has bicycling grown with the introduction of CaBi?”

The real answer to that question will come through household travel survey data releases that incorporate CaBi’s lifespan (in other words, data from calendar year 2011 and onward). But in the meantime, we can easily figure out a benchmark number to frame our expectations. Back-of-the-envelope time!

It is pretty easy to find out what proportion of trips are by bicycle, via national and regional household travel surveys. What I had a devil of a time finding what was finding out the actual number of trips. Which I never did. So, here’s my imperfect best guess (save your receipts for a full refund):

About 215,000 people live in Arlington County, VA. About 615,000 live in Washington, DC.

While people in other surrounding counties also ride CaBi, we’re going to assume that it’s in much smaller numbers than the two core jursidictions, and we are going to ignore them, lest it skews this slipshod analysis. Sorry, Marylanders.

People in the survey averaged about 4 trips per person per day.

Based on that, we can estimate (people x trips x bike rates = total trips) that somewhere around 44,000 combined trips are taken by Arlington and District residents by bike every day.

According to the latest CaBi Dashboard ridership data, CaBi averaged 4,715 rides per day over the past year. That, assuming that 100% CaBi trips are additive, is just under an 11% bump in bicycle ridership directly attributable to CaBi. We do not know bicycle ridership for 2009 or 2010 (the year CaBi rolled out), though we can assume that it followed recent trends in ridership and population growth, and rose further beyond the 44K trip level.

For somebody who is an unabashed lover of bikesharing, an 11% bump does not seem to reflect the dramatic change that CaBi has anecdotally brought to bicycling in the region. To reach the levels of bicycle modeshare in Europe, we need to multiply by whole numbers.